Thin line at left of cells in selected row in tstringgrid which is different color than

Question

I have a tstringgrid on a form which uses this code to highlight a selected row:

procedure TTestForm.testGridDrawCell(Sender: TObject; ACol,
  ARow: Integer; Rect: TRect; State: TGridDrawState);
begin

  testGrid.Canvas.Pen.Color := clNone;


  if ARow <> SelectedRow then
    testGrid.Canvas.Brush.Color := clWindow
  else testGrid.Canvas.Brush.Color := clActiveCaption;


  if ColorTSTringList[ARow] = 'Red' then
    testGrid.Canvas.Font.Color := clRed;

  testGrid.Canvas.Rectangle(Rect.Left, Rect.Top, Rect.Right, Rect.Bottom);

  testGrid.Canvas.TextRect(Rect, Rect.Left,Rect.Top,testGrid.Cells[ACol,ARow]);


end;

For some reason, there is a slightly darker blue rectangle, perhaps 3 pixels wide and the height of the cell in each row which is highlighted (see attached image). Any ideas on this one?